There is a present day need in motion picture projectors used in theatres for providing a very fast pull down or transport of the film with an accompanying long dwell period, preferably 270.degree. over a 360.degree. cycle. In conventional motion picture theatre projectors, the old proven Geneva movement is presently used for intermittently moving appropriate pull down sprockets. While these movements have great reliability, the Geneva movement itself has a somewhat abrupt acceleration and deceleration characteristic, which imparts cumulative stresses on both the film, film sprockets and other mechanism. Moreover, the strong vibrations of free or unsupported film loops associated with conventional film movements cause noise and instabilities which become increasingly objectionable with larger film formats.
It is recognized, that if projection equipment can be provided which avoids these problems, film copies can be run more often in theatres and substantial savings will result.
In the past there have been a number of proposed constructions for eliminating free loops in motion picture equipment. Essentially, these systems have captive loops which have a number of benefits to be described subsequently. However, the systems have shortcomings when applied to motion picture projector uses. Interjected spring-loaded members to hold the film loops captive have proven unreliable and introduce other problems.
The problem has been recognized for some time as evidenced by U.S. Pat. Nos. 629,582 issued July 25, 1899; 704,449 issued July 8, 1902, and 2,039,906 issued May 5, 1936. These three prior art patents are examples of captive loop constructions, but the features therein have not been carried sufficiently forward to provide a practical and workable system. For example, adequate motion control systems are not evident that will assure a complete captive loop operation throughout an entire cycle. Further, long dwell times have not been achievable with these prior art systems. A more recent U.S. Pat. No. 3,613,978, issued Oct. 19, 1971 provides for a properly captive film but a dwell time of only one-fourth the entire cycle. Such known systems are not suitable for use in projection equipment requiring long dwell periods of up to or even greater than 270.degree..